One of the most intriguing and important (from an applicative point of view) properties of \(C_{60}\)  is related to its optical spectrum. Being a carbon compound, one could hardly view it as a metallic cluster or think about metallic properties. Even so, since the first theoretical calculations [1]} and spectroscopic experiments [2]}, a giant resonance has been calculated and observed around \(20 eV\)  in its photoionization spectrum. It can be interpreted accordingly with the Mie's theory [3]} as a surface plasmon which is usually present in metal nanostructures where the electrons have the characteristic of being quasi-free so the response function to external fields is caracterized by high values and a strong collective behavior. This aspect can be explained from the electronic structure \(1s^22s^22p^2\)  of carbon atoms, imagining that the valence electrons \(2s^22p^2\)  are quasi-delocalized in the cluster structure and capable of being collectively excited. Even peculiar was the presence of a second smaller peak in the optical spectrum around \(40 eV\)  revealed in photoionization experiments [4]},[5]} both for neutral or ionized fullerenes. In the past decade, a lot of studies have reproduced it theoretically and interpreted it as a compressional collective mode associated with the so called volume plasmon. Also, regarding its microscopical nature, pure quantum effects are thought to be responsible through a coherent superposition of orbital excitations. Some other studies, based on hydrodynamical pictures have argued that its nature is also that of a surface plasmon due to oscillations from the inner and outer surfaces of the electron cloud. Actually, one can find an impressive collection of papers regarding the existence and the nature of second resonance, but still there are debates about the correct interpretation.
